Polyamides are synthetic polymers where the repeating units (monomers) possess the amide group as a characteristic feature. The designation “polyamides” is usually used to designate synthetic, commercially usable thermoplastics and therefore demarcates this class of substances from the chemically related proteins. Nearly all the important polyamides are derived from primary amines, i.e. the functional group —CO—NH— occurs in their repeat units. Polyamides of secondary amines (—CO—NR—, R=organic residue) also exist. Aminocarboxylic acids, lactams and/or diamines and dicarboxylic acids in particular find application as monomers for the polyamides.
When the repeating units between the amide linkages are substantially aliphatic, they form polyamide polymers that are called nylon. Nylon is known to be made through the condensation reaction between a diamine and a diacid. Nylon is one of the most widely used polymers because of its characteristics. In particular, nylon is highly resilient and durable. Nylon fibres are thus used in many applications, including clothes fabrics, package paper, carpets, musical strings, pipes, rope, mechanical parts and the like.
Some commonly available nylon polymers include but are not limited to nylon 6,6, nylon 6, nylon 11, nylon 12, nylon 4,6, nylon 6,12, nylon 6,10 and the like. Out of which, nylon 6,6 and nylon 6 are the more commonly used polymers. Nylon may be made through step-growth polymerisation or chain-growth polymerisation. Making nylon from a diamine and a diacid or an amino acid is a step-growth polymerisation whereas making nylon from lactams usually involves a chain-growth polymerisation. Since the latter method starts off with monomers, the monomers quickly form high molecular weight polymers making the process of polymerisation more efficient. This method also reduces the formation of intermediate dimers, trimers, and other oligomers. This method is thus more favoured than step-growth polymerisation.
Caprolactam is the feedstock in the production of nylon 6 using chain-growth polymerisation. The production of caprolactam is usually carried out by reacting cyclohexanone with hydrogensulphate or hydrochloride of hydroxylamine resulting in the formation of cyclohexanone oxime. This is then converted by a Beckmann rearrangement into caprolactam, often with the use of concentrated sulphuric acid as catalyst. The raw material, cyclohexanone is usually produced by catalytic oxidation of cyclohexane with oxygen of the air and cyclohexane is in turn obtained by hydrogenation of benzene.
There are several disadvantages with the currently available methods of producing nylon. One disadvantage in the production of lactams by Beckmann rearrangement of oximes is, among other things, that large amounts of salts, for example sodium sulphate, are formed as by-product, which requires disposal. Other methods for the production of lactams described in the prior art which use a different method include EP0748797 which describes a method of production of lactams from dinitriles, in which the dinitrile is hydrogenated to aminonitrile and the aminonitrile is converted by cyclizing hydrolysis to the lactam. Molecular sieves, such as acid zeolites, silicates and non-zeolitic molecular sieves, metal phosphates and metal oxides or mixed metal oxides were used as catalyst for cyclizing hydrolysis. However, this method has, among other drawbacks, the disadvantage that the selectivity of the conversion of the aminonitrile by cyclizing hydrolysis is rather low and therefore large amounts of by-products are also formed.
Further, the traditional manufacture of nylon uses petroleum based intermediates. For example, cyclohexane is used to make adipic acid and caprolactam. Butadiene and natural gas are important raw materials for making hexamethylene diamine. Nylon 12 is also dependent upon butadiene feedstocks. Thus, in most methods of producing lactams described in the prior art, hydrocarbons such as benzene or butadiene are used, and these are obtained by cracking gasoline or petroleum which is bad for the environment. Also, since the costs for these starting materials will be linked to the price of petroleum, with the expected increase in petroleum prices in the future, nylon prices may also increase relative to the increase in the petroleum prices.
Accordingly, it is desirable to find more sustainable raw materials, other than purely petroleum based, as starting materials for nylon production which also cause less damage to the environment.